Pass-through metal detectors are used in the security access control field to screen people and objects passing through a portal at airports, courts, prisons, passenger ships, nuclear facilities, and other high-security areas, as well as to screen against employee theft, contamination and other purposes. Known metal detectors generally include one transmitter coil of one or more windings in an elongated symmetrical shape located in a panel on one side of the portal. The transmitter coil generally generates a pulsed magnetic field, which excites eddy currents in metal objects passing through the portal. Located in a panel on the opposite side of the portal is either one or a pair of receiver coils which are similar in size and shape to the transmitter coil. These receiver coils receive the magnetic field created by the eddy currents in the metal objects passing through the portal, as well as the magnetic field created by the transmitter coil.
A receiver antenna may include two identical receiver coils disposed in essentially the same plane and wound in opposite directions so that any signal that has equal amplitude through both coils, such as some forms of signal noise, are subtracted. This arrangement significantly reduces noise. The output of the receiver coils is connected to a resistor-capacitor circuit that creates a damped oscillating signal at the end of each transmitter pulse. A control circuit then processes this signal to amplify the eddy currents and to detect variations in the magnetic field caused by the moving metal. The circuit continuously compares the signal level to a threshold set by the operator, and triggers an alarm when the threshold is exceeded. The panels containing the coils may be connected at their tops with a crosspiece wide enough to permit passage.
Sheets of conductive metal (shields) are sometimes used on the outside of metal detectors to better prevent metal objects that are moving on the outside of the portal from being detected. The large eddy currents created in the metal shields can be much larger than the signal from the metal objects themselves, and can change the balance of the two coils, causing a very large output signal and saturation of the circuit.
Referring to FIG. 1, there is shown a schematic diagram of a metal detector of the prior art. Two receiver coils L1 and L2 are connected at their inner terminals to form a dual-coil receiver antenna. An outer terminal of receiver coil L1 is connected to an outer terminal of a resistor R1, a first terminal of a capacitor C1, and to a first input of a differential amplifier 1. Similarly, an outer terminal of receiver coil L2 is connected to an outer terminal of a resistor R2, a second terminal of capacitor C1, and to a second input of differential amplifier 1. The inner terminals of resistor R1 and resistor R2 are connected to each other, as well as to the inner terminals of receiver coil L1 and receiver coil L2. Together, the two receiver coils L1 and L2, resistors R1 and R2, capacitor C1 and differential amplifier 1 form a receiver 102.
An output of differential amplifier 1 is connected to an input of a control unit 2, an output of which is connected to an input of an amplifier 3. Finally, each of two outputs of amplifier 3 is connected to a respective terminal of a transmitter coil (transmitter antenna) L3. Together, the transmitter coil L3 and amplifier 3 form a transmitter 101.
The two receiver coils L1 and L2 are wound in opposite directions so that signals going through one coil are subtracted from signals going through the other coil. Because most electrical noise sources are a relatively long distance away, the noise through receiver coil L1 is approximately equal to the noise through receiver coil L2, i.e., the difference is approximately zero, and thus noise is reduced. This system of noise reduction operates without any active components and with signals of any amplitude.
The resistor-capacitor (RC) network including R1, R2, and C1 forms an oscillating circuit, which transmits a damped oscillating signal to differential amplifier 1. The oscillation starts when a pulse from the transmitter coil L3 ends. Amplifier 3 provides a pulse to transmitter coil L3 to form a magnetic field which creates eddy currents in a metal object (not shown) passing through a detection area 4. The control unit 2 performs various control functions, such as processing signals from the receiver, and controlling the transmitter timing.
Referring now to FIG. 2, there is shown a portion of the metal detector of the prior art of FIG. 1 disposed in a portal. The portal 201 includes a side panel 202 and a side panel 203, which are connected at the top by a crosspiece 204 wide enough to permit passage of a person. Side panel 202 contains the transmitter coil L3, while side panel 203 contains the two receiver coils L1 and L2.
A two-sided metal detector is known from U.S. Pat. No. 4,779,048, which describes a metal detector in which field coils and receiving coils are arranged on either side of a zone to be monitored. The field coil on one side and the field coil on the other side are alternately excited for radiating a magnetic field.
One problem with this system is that the indiscriminate placement of a transmitter antenna and a receiver antenna in a transmitter-receiver pair will cause the receiver to receive a much larger signal from the adjacent transmitter (i.e., the transmitter in the pair) than from the opposite transmitter, undesirably affecting the receiver's output. The indiscriminate placement may even create high output voltages caused by a field through one of the receiver coils being much larger than a field through the other receiver coil, so that the voltage difference between the coils is very large.
This system may attempt to overcome this high noise problem through the use of an analog switch (not shown). However, doing so chops the receiver signal and causes harmonics that affect the filtering of the signal. Moreover, when the receiver voltage is several volts, saturation and even damage to the system is possible. In order for damped oscillations to decrease to a safe level, a delay time would be too large to allow timely acquisition of the signal. High cost and resistance of the switch also cause problems with this technique. The system is very noisy, and may exhibit especially low detection in a zone approximately midway between the sides of the passage, resulting in a high false alarm rate.
It is an object of the present invention to provide a metal detector system and method of calibrating and operating a metal detector system which allows more information to be cleanly sensed and processed, among other advantages.